Synthesis of photocleavable poly(methyl methacrylate-block- d -lactide) via atom-transfer radical polymerization and ring-opening polymerization

Article abstract of DOI:10.1002/pola.26840

The synthesis and characterization of a photocleavable block copolymer containing an ortho-nitrobenzyl (ONB) linker between poly(methyl methacrylate) and poly(d-lactide) blocks is presented here. The block copolymers were synthesized via atom transfer rad

Full text of DOI:10.1002/pola.26840

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Synthesis of Photocleavable Poly(methyl methacrylate-block-D-lactide) via
Atom-Transfer Radical Polymerization and Ring-Opening Polymerization
Hong Li,1 Sahas Rathi,2 Elizabeth S. Sterner,2 Hui Zhao,3 Shaw Ling Hsu,2 Patrick Theato,3
Yongming Zhang,1 E. Bryan Coughlin2
1School of Chemistry and Chemical Engineering, Shanghai Jiaotong University, Shanghai 200240, People’s Republic of China
2Department of Polymer Science and Engineering, University of Massachusetts Amherst, Massachusetts 01003
3Institute for Technical and Macromolecular Chemistry, University of Hamburg, Hamburg D-20146, Germany
Correspondence to: H. Li (E-mail: lh102@sjtu.edu.cn) or E. B. Coughlin (E-mail: Coughlin@mail.pse.umass.edu)
Received 8 April 2013; accepted 17 June 2013; published online 00 Month 2013
DOI: 10.1002/pola.26840
ABSTRACT: The synthesis and characterization of a photocleavable
block copolymer containing an ortho-nitrobenzyl (ONB) linker
between poly(methyl methacrylate) and poly(D-lactide) blocks is
presented here. The block copolymers were synthesized via atom
transfer radical polymerization (ATRP) of MMA followed by ring-
opening polymerization (ROP) of D-Lactide and ROP of D-lactide fol-
lowed by ATRP of MMA from a difunctional photoresponsive ONB
initiator, respectively. The challenges and limitations during syn-
thesis of the photocleavable block copolymers using the difunc-
tional photoresponsive ONB initiator are discussed. The
photocleavage of the copolymers occurs under mild conditions by
simple irradiation with 302 nm wavelength UV light (Relative inten-
2
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sity at 7.6 cm: 1500 lW/cm ) for several hours. 2013 Wiley Period-
icals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 00, 000–000
KEYWORDS: atom transfer radical polymerization (ATRP); block
copolymers; D-lactide; photochemistry; photocleavable; photo-
reactive effects; ring-opening polymerization
Herein, poly(D-lactide)-b-poly(methyl methacrylate) and
INTRODUCTION Photolabile groups, such as ortho-nitroben-
zyl (ONB) alcohol derivatives,1–3 ketoester derivatives,4,5
and benzophenone esters,6 have been used extensively in
synthetic organic chemistry and have found numerous
applications in academia and industry. Among the many
photolabile groups that have been studied, ONB alcohol
derivatives have gained tremendous attention and have
become one of the most popular photolabile protecting
groups.7,8 ONB structures have been used as crosslinkers
for photodegradable hydrogels,9–14 as photocleavable link-
ers in block copolymers,15–22 in photocleavable bioconju-
gates,23–25 and for side-chain functionalization in
copolymers.26–28 These cleavable materials are particularly
useful as precursors for the production of highly ordered
nanoporous templates by facilitating the removal of the
minor block of phase-separated block copolymer films, and
producing light-sensitive block copolymer micelles. Many
block polymers, for example, polystyrene-b-poly(methyl
methacrylate) (PS-b-PMMA),19 polystyrene-b-poly(ethylene
oxide) (PS-b-PEO),19,20 poly(c-methyl-e-caprolactone)-b-
poly(acrylic acid) (PmCL-b-PAA),21 polystyrene-b-poly(e-
caprolactone) (PS-b-PCL),22 and polyurethane-b-poly(ethyl-
ene oxide) (polyurethane-b-PEO) have been synthesized
with photocleavable ONB junctions.29
poly(methyl methacrylate)-b-poly(D-lactide) containing
a
photocleavable ONB group as linker (abbreviated:
a
PDLA-ONB-PMMA/PMMA-ONB-PDLA) were successfully syn-
thesized by combining atom transfer radical polymerization
(ATRP) and ring-opening polymerization (ROP) for the first
time. The challenge and limitation during synthesis of the
block copolymers using the ONB initiator including nitroben-
zene for ATRP and the basic condition for ROP were high-
lighted. We use a biocompatible and biodegradable PDLA
sacrificial block, which is also suitable for bio-related appli-
cations.30,31 In addition, it is well recognized that ultraviolet
(UV) irradiation induces important physico-chemical changes
in PLA polymers involving a reduction of molecular weights,
which will benefit facile removal of the sacrificial block
(PDLA) after cleavage.32–37
EXPERIMENTAL
Materials
All commercially obtained solvents and reagents were used
without further purification except as noted below. D-Lac-
tide was kindly provided by Purac Biochem BV (Gorinchem,
The Netherlands), and recrystallized three times from dry
toluene. Methyl methacrylate (MMA) was washed with
Additional Supporting Information may be found in the online version of this article.
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5 wt % sodium hydroxide (NaOH) aqueous solution to
remove the inhibitor and washed with water several times.
The collected organic layers were dried with anhydrous
magnesium sulfate (MgSO4) overnight and vacuum distilled
before use. Tin (II) 2-ethylhexanoate [Sn(Oct)2] was dried
under high vacuum at 80 ꢀC overnight before use. 1,8-Dia-
zabicyclo [5.4.0] undec-7-ene (DBU) was vacuum distilled
from calcium hydride (CaH2) before use. Toluene and tetra-
hydrofuran (THF) were distilled from sodium (Na)/benzo-
phenone. Dry dichloromethane (CH2Cl2) was obtained by
distilling from CaH2.
General Procedures for the Synthesis of Block
Copolymers PDLA-ONB-PMMA (Route A, ROP Followed
by ATRP)
ROP was performed under inert atmosphere and water-
free conditions. D-Lactide (0.5 g, 3.47 mmol), ONB (0.1 g,
0.314 mmol), and freshly distilled toluene (5 mL) were
added to
a flame-dried two-neck, round-bottom flask
equipped with a gas inlet and a septum under dry N2
flow. A solution of Sn(Oct)2 (0.01 g) in toluene (ꢁ0.1 mL)
was added via gas-purged syrꢀinge. The solution was
allowed to react for 3 days at 80 C. The resulting polymer
was precipitated from 150 mL of hexane, and washed with
cold methanol. The polymer was dried over night at 50 ꢀC
under high vacuum.
Synthesis of Difunctional Photocleavable Initiator ONB
5-Hydroxy-2-nitrobenzyl Alcohol
In a round-bottom dry flask, 5-hydroxy-2-nitrobenzaldehyde
(10.1 g, 0.06 mol) was dissolved in anhydrous methanol
(MeOH) (150 mL). Sodium borohydride (NaBH4) (6 g, 0.16
mol) was slowly added under stirring and nitrogen flow at
For the ATRP of MMA under oxygen-free conditions, macroi-
nitiator ONB-PDLA (0.2 g, 0.11 mmol, Mn,NMR 5 1760 g/mol,
Ð 5 1.20), MMA (0.51 g, 5.1 mmol), CuBr (0.016 g, 0.11
mmol), and freshly distilled toluene (2.5 mL) were added to
a double-neck, round-bottom flask equipped with a gas inlet
and a septum. The solution was degassed with three freeze/
pump/thaw cycles. N,N,N0,N00,N00-pentamethyldiethylenetri-
amine (PMDETA) (0.029 mL, 0.11 mmol) was added via gas-
purged syringe to the reaction flask. The flask was then
placed in an oil bath at 80 ꢀC. After a given reaction time,
the resulting polymer solution was cooled, exposed to air,
and diluted with THF. The polymer solution was passed
through a neutral alumina column. After concentration, the
polymer was precipitated into 100 mL of cold methanol and
dried under high vacuum.
0
ꢀC. Then, the resulting mixture was warmed to room tem-
perature and stirred for 3 h. The reaction was cautiously
quenched by the addition of a 10% HCl (aq.) solution, and
extracted with ethyl acetate (EtOAc) (200 mL 3 3). The com-
bined organic layers were washed with brine, dried with
anhydrous MgSO4, and concentrated in vacuum. The yellow
solid residue was purified by flash column chromatography
with hexane/EtOAc (1:1 v:v) to give 5-hydroxy-2-nitrobenzyl
alcohol as a yellow solid (yield, 94%).
1H NMR (300 MHz, DMSO-d6, d ppm): 10.89 (s, 1H), 8.07
(d, 1H), 7.29 (s, 1H), 6.81 (d, 1H), 5.52 (s, 1H), 4.85
(s, 1H).
5-Bromoisobutyrate-2-nitrobenzyl Alcohol
General Procedures for the Synthesis of the Block
Copolymers PMMA-ONB-PDLA (Route B, ATRP Followed
by ROP)
In a round-bottom flask, 5-hydroxy-2-nitrobenzyl alcohol
(4 g, 0.024 mol) was dissolved in anhydrous THF (20 mL).
A sodium hydride (NaH) slurry in dry THF (20 mL, 0.024
mol NaH) was added slowly under stirring and nitrogen
flow. a-Bromoisobutyryl bromide (6 g, 0.026 mmol) was
added dropwise via a constant pressure funnel at 0 ꢀC.
Then, the reaction mixture was warmed to room tempera-
ture and stirred overnight. The solution was diluted with
twice its volume in EtOAc. The organic phase was washed
with 1 M of NaOH (aq) (2 3 100 mL), brine (2 3 100 mL),
and water (3 3 100 mL), then dried over MgSO4 and the
solvent was evaporated under reduced pressure. The crude
product was purified by chromatography with hexane/
EtOAc (1:2 v:v) to yield compound ONB as a clear yellow
oil. The yield of ONB is 65%, covering both the installation
of the ATRP initiating site and the sodium borohydride
reduction.
For the ATRP of MMA under oxygen-free conditions, ONB
initiator (0.34 g, 1.1 mmol), MMA (5.7 g, 57 mmol), CuBr
(158 mg, 1.1 mmol), and freshly distilled toluene
(30 mL) were added to a two-neck, round-bottom flask
equipped with a gas inlet and a septum. The solution
was degassed with three freeze/pump/thaw cycles.
PMDETA (0.23 mL, 1.1 mmol) was added via gas-purged
syringe to the reaction flask. The flask was then placed
in an oil bath at 80 ꢀC. After 5 h resulting polymer solu-
tion was cooled, exposed to air and diluted with THF.
The polymer solution was passed through
a neutral
alumina column. After concentration, the polymer was
precipitated from 100 mL of cold methanol and dried
under high vacuum.
ROP of D-lactide was performed under inert atmosphere
and water-free conditions. D-Lactide (0.25 g, 1.73 mmol),
macroinitiator ONB-PMMA (0.208g, Mn,nmr 5 15,300
g/mol, Ð 5 1.19) and freshly distilled toluene (4 mL)were
added to a flame dried two-neck, round-bottom flask
equipped with a gas inlet and a septum under N2 flow. A
solution of Sn(Oct)2 (0.007 g) in toluene was added via
gas-purged syringe. The solution was allowed to react for
3 days at 80 ꢀC. The resulting polymer solution was
1H NMR (CDCl3, 300 MHz) d (ppm): 2.12 (6H, s, 23 CH3),
5.08 (2H, d, CH2), 7.25 (1H, dd, aromatic), 7.61 (1H, d, aro-
matic), 8.22 (1H, d, aromatic); 13C NMR (300 MHz, MeOH-
d4): d 171.14 (C@O), 156.32 (CArANO2), 145.97 (CArAO),
142.62 (CArACH2), 128.08 (CAr), 122.20 (CAr), 61.80 (CAr),
56.77 (CH2AOH), 31.88 (CA (CH3)2), 30.76 (CH3). FTIR
(ATR, cm21): 3546 (b, OH), 1756 (s, C@O), 1584 (m, NO2),
1521 (s, NO2), 1215 (s, CABr), 1128 (s, ArAOACA).
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SCHEME 1 Synthesis of photocleavable poly(methyl methacrylate-b-D-lactide).
precipitated from 150 mL of hexane and washed with
cold methanol. The polymer was dried overnight at 50 ꢀC
under high vacuum.
Characterization
1H NMR spectra were recorded on a Bruker DPX-300 NMR
Spectrometer in chloroform-d (CDCl3), except where other-
wise noted. Molecular weight and dispersity (Ð) were meas-
ured on an Agilent Technologies 1200 gel permeation
chromatograph (GPC) in CHCl3 at 40 ꢀC with a flow rate of
1 mL/min on systems equipped with three-column sets
In the case of DBU as catalyst, D-lactide (0.25 g, 1.73 mmol),
macroinitiator ONB-PMMA (0.208 g, Mn,nmr 5 15,300 g/mol,
Ð 5 1.19) and freshly distilled CH2Cl2 (2 mL)were added to a
flame dried two-neck, round-bottom flask equipped with a
gas inlet and a septum under N2 flow. DBU (4 lL, 0.0268
mmol) was added via gas-purged micro syringe. The solution
(Polymer Laboratories, 300 mm 3 7.5 mm, 5 lm, 1025
,
1024, and 1023 Å pore sizes) and refractive index detectors
(HP 1047A) at 40 ꢀC. Linear polystyrene standards were
used for molecular weight calibration. A UVM-57 handheld
UV lamp (302 nm, 6 W, 0.16 Å, Relative intensity at 7.6 cm:
1500 lW/cm2) was used for photocleavge. UV–Vis spectra
were acquired on a Perkin-Elmer Lambda 35 instrument at
room temperature.
ꢀ
was allowed to react at 25 C for 4 h and then quenched by
injecting 1 mL of methanol. The resulting polymer solution
was precipitated from 150 mL of hexane and washed with
cold methanol. The polymer was dried overnight at 50 ꢀC
under high vacuum.
FIGURE 1 1H NMR spectra of ONB-PDLA20 (a) and PDLA20-ONB-PMMA93 (b) in CDCl3.
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TABLE 1 Molecular Weights and Compositions of the ONB
shown in Scheme 1: (A) synthesis of an ONB-poly(D-lac-
tide) (ONB-PDLA) macroinitiator by ROP of D-lactide and
followed by chain extension of ONB-PDLA by ATRP of
MMA; or (B) preparation in the reverse manner, that is,
ATRP of MMA with subsequent chain extension by ROP of
D-lactide.
Macroinitiators and Synthesized Block Copolymers
Mn,NMR
Mn,GPC
(kg/mol)
(kg/mol)
Ð
ONB-PDLA20
1.76
3.20
11.4
11.8
19.1
15.3
32.6
21.8
23.3
21.4
1.7
3.3
19
12
15
14
20
16
20
18
1.20
1.25
1.23
1.19
1.18
1.19
1.55
1.34
1.24
1.23
ONB-PDLA40
Route A: ROP of D-Lactide and Followed by Chain
Extension of ONB-PDLA by ATRP of MMA
PDLA20-ONB-PMMA93
PDLA40-ONB-PMMA79
ONB-PMMA188
For ROP of D-lactide, solution or bulk polymerization meth-
ods can be used. The former can be performed at relatively
low temperatures but require longer reaction times, wheras
the latter need higher temperatures with shorter reaction
times. Here, to protect the a-bromoester functional group of
the bifunctional initiator ONB from dissociation at high tem-
peratures, the solution polymerization method was applied.
ROP of D-lactide was performed at 80 ꢀC using tin (II)
bis(2-ethylhexanoate) [Sn(Oct)2] as catalyst and toluene as
solvent. Figure 1(a) shows the 1H NMR spectrum of the
obtained polymer ONB-PDLA. We calculated the degree of
polymerization by comparing the integral of the nitrobenzyl
protons (d) with the methine protons (f) of the ONB-PDLA.
This calculated value is in good agreement with the theoreti-
cal values calculated from the monomer to initiator molar
ONB-PMMA150
PMMA188-ONB-PDLA183-Sna
PMMA188-ONB-PDLA33-Sna
PMMA188-ONB-PDLA138-DBUb
PMMA188-ONB-PDLA32-DBUb
a
The block copolymer was synthesized using Sn(Oct)2 as catalyst and
ONB-PMMA188 as macroinitiator.
b
The block copolymer was synthesized using DBU as catalyst and
ONB-PMMA188 as macroinitiator.
RESULTS AND DISCUSSION
1
The bifunctional ONB initiator, 5-bromoisobutyrate-2-
nitrobenzyl alcohol, bearing a hydroxyl group and a a-
bromoester group was prepared in two steps. The first is
that 5-hydroxy-2-nitrobenzaldehyde (1) was reduced with
NaBH4 to give 5-hydroxy-2-nitrobenzyl alcohol (2). Then,
5-hydroxy-2-nitrobenzyl alcohol was esterified with
a-bromoisobutyryl bromide. As the phenol of 5-hydroxy-2-
nitrobenzyl alcohol is several orders of magnitude more
reactive for nucleophilic substitution than the benzyl alco-
hol, particularly as there is the nitro group para to the
phenol. With proper stoichiometric control of the deproto-
nating agent, and slow addition of a slight excess of the
ratio. The molecular weights from GPC and H NMR and the
Ð of ONBPDLA are listed in Table 1.
ROP of D-lactide was also attempted by organo-base catalysis
with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which has
been demonstrated to work well for lactide ring opening
with superb end group-fidelity, good polymerization control,
and fast kinetics at room temperature.38 However, in our sys-
tem, no ONB-PDLA polymer was obtained using DBU as cata-
lyst and ONB as initiator at 25 ꢀC after 4 h. The 1H NMR
spectrum of the mixture of ONB and DBU shows that ONB is
not stable in the presence of DBU (Supporting Information
Fig. S1). The complex nature of the NMR spectrum indicates
the presence of several species owing to the occurrence of
side reactions. As no polymer was obtained, our hypothesis
is that transesterification at the ATRP initiating site occurred,
and thus destroying it.
2-bromoisobutyryl bromide,
a
very high degree of
functionalization only at the phenol can be achieved. Con-
sidering the bifunctional initiator ONB used for the syn-
thesis of the targeted block copolymers, two different
synthetic routes appeared to be possible and both are
FIGURE 2 GPC traces of ONB-PDLA macroinitiator and the corresponding diblock copolymer PDLA-ONB-PMMA.
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Route B: ATRP of MMA with Subsequent Chain Extension
by ROP of D-Lactide
ATRP of MMA initiated by ONB was conducted at 80 ꢀC
using distilled toluene as solvent. Figure 3 shows the results
of the kinetic studies on ATRP of MMA using ONB as the ini-
tiator. Compared to the kinetics curve of ATRP of MMA using
a-bromoisobutyrate as initiator,39 we found that when using
ONB as initiator, the polymerization rate is slower and
monomer conversion is much lower. The nonlinear plot of
ln[M]/[M0] versus time (Fig. 3) indicates that some side
reaction and/or termination occurred in the system. In fact,
the nitrobenzene group of the initiator has been shown to
perturb radical polymerization.40 Although the molecular
weight cannot be controlled well, the obtained ONB-PMMA
has a fairly narrow Ð (Table 1). In the kinetics study, low
MMA initial concentration (0.13 g/mL) was used owing to
easy to withdraw the samples from the flask and the eval-
uated sample reached a conversion of only 32% after 8 h.
Increasing the monomer concentration could possibly
improve the polymerization rate and monomer conversion.
With increasing monomer concentration to 0.19 g/mL, the
FIGURE 3 Kinetics study for ATRP of MMA using ONB as
initiator.
The completion of the ATRP of MMA initiated by ONB-
PDLA is confirmed via 1H NMR with the appearance of
characteristic resonances belonging to the methoxy (j) and
methyl groups (k) of the PMMA side chain, and methylene
(l) and methine (o) groups of the PMMA backbone [Fig.
1(b)]. The GPC curves of the copolymer PDLA-ONB-PMMA
(Fig. 2, red lines) reveal a clear shift toward higher MW
when compared to the ONB-PDLA macroinitiator trace (Fig.
2, black lines). Moreover, the diblock copolymer traces are
unimodal and symmetrical, no tailing in the low-molecular-
weight region is observed, and the molecular weight distri-
butions are quite narrow. The molecular weights from GPC
and 1H NMR and the Ð of the PDLA-ONB-PMMA are listed
in Table 1. The repeat units of both blocks were calculated
conversion of MMA after
8 h can rise to 78.5%.
ONB-PMMA188 with narrow dispersity is synthesized under
this condition. However, its molecule weight (Mn,GPC) is much
higher than theory one. The initiating efficiency calculated
from Mn,GPC/Mn,theory is about 30%, which indicates that the
polymerization is not well controlled and nitrobenzene
group in ONB initiator perturbs ATRP.
Figure 4(a) shows the 1H NMR spectrum of ONB-PMMA. In
addition to the characteristic protons of the methoxy, methyl-
ene, and methine groups in PMMA, the protons from the
nitrobenzene group are apparent. The molecular weight of
from the Mn,NMR
.
FIGURE 4 1H NMR spectra of ONB-PMMA188 (a) and PMMA188-ONB-PDLA138-DBU (b) in CDCl3.
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macroinitiator for ROP of D-lactide using the same catalyst.
This may be owing to the PMMA macromolecular chain steri-
cally protecting the ONB ester from transesterification or
other side reactions.
Photocleavage of the Block Copolymers
The block copolymers PMMA188-ONB-PDLA138-DBU and
PMMA188-ONB-PDLA183-Sn were dissolved in THF to prepare 1
mg/mL solutions, and the solutions were exposed to 365 nm
UV light (UVL-56, 1350 lW/cm2 at 7.6 cm, 583 J/cm2) for 12
h. The cleaved polymer was isolated by simply removing the
solvent in vacuum. GPC analysis of the polymer after UV expo-
sure shows that the elution time of the sample is almost same
as that before UV exposure and no drop in molecular weight
occurred, which means that photocleavage reaction did not
occur under these conditions. In addition, a low-molecular-
weight (Mn 5 300 g/mol) peak appeared near the toluene flow
marker. The UV–Vis spectrum of ONB initiator is shown in Sup-
porting Information Figure S3. The maximum absorption of
ONB occurred at about 300 nm. According to the UV–Vis spec-
tra of the copolymer PMMA188-ONB-PDLA138-DBU (Fig. 5), the
maximum absorption of the copolymer occurred at about 290–
300 nm and there is very little absorption at 365 nm. There-
fore, a UV lamp with its maximum emission at 302 nm and
relative intensity at 7.3 cm of 1500 lW/cm2 was used instead
for the photocleavage reaction. The evolution of the UV absorp-
tion spectrum with exposure time is shown in Figure 5. A red-
shifted absorption is observed for the diblock copolymer after
UV exposure, indicating that the ONB linker is cleaved and
transforms from a nitrobenzyl linker to the nitrosobenzalde-
hyde coproduct (Scheme 1). The THF solution of the copoly-
mer after UV exposure was evaporated and washed with
MeOH. GPC analysis of the copolymers before, and after, UV
irradiation is shown in Figure 6. After irradiation, the peak
associated with the block copolymer is no longer observed,
and one new peak is visible on the chromatogram, correspond-
ing to the dissociated PMMA block. Meanwhile, the expected
PDLA block is not observed in the GPC curve. When the
polymer after UV exposure was not washed by MeOH, the
peak corresponding to PMMA block is accompanied by
FIGURE 5 Time-dependent UV–Vis spectra changes of
PMMA188-ONB-PDLA138-DBU in THF (0.86 mg/mL).
ONB-PMMA was calculated according to the integration of
the aromatic proton near the nitro group and the methoxy
protons in the PMMA. This value agrees with the molecular
weight obtained by GPC.
Chain extension of ONB-PMMA was performed by ROP of
D-lactide. Here, both Sn(Oct)2 and DBU catalysts were used,
as either catalyst system worked well. The obtained copoly-
mers were characterized by 1H NMR [Fig. 4(b]) and GPC
(Table 1). Compared with the 1H NMR spectrum of ONB-
PMMA, two new resonances (at 5.2 and 1.7 ppm) appeared
in the 1H NMR spectrum of PMMA-ONB-PDLA. These corre-
sponded to the methine proton and methyl protons of PDLA,
respectively. The copolymers obtained from Sn(Oct)2
(PMMA188-ONB-PDLA183-Sn and PMMA188-ONB-PDLA33-Sn)
have broader Ð than those of the copolymers obtained
with the DBU catalyst (PMMA188-ONB-PDLA138-DBU and
PMMA188-ONB-PDLA32-DBU), which may be owing to the
occurrence of transesterification reactions when the Sn(Oct)2
catalyst is used. Although ONB cannot initiate ROP of
D-lactide using DBU as catalyst, ONB-PMMA can act as
FIGURE 6 GPC traces of PMMA188-ONB-PDLA138-DBU (left) and PMMA188-ONB-PDLA183-Sn (right) before and after UV exposure
(302 nm) for several hours.
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FIGURE 7 1H NMR spectra of PDLA20-ONB-PMMA93 before (a) and after (b) UV exposure.
one more peak at about 300 g/mol (Supporting Information
Fig. 2S). This peak might arise from the photodegradation of
the PDLA block. The photodegradation of PLA is reported to
proceed via the Norrish II mechanism at ester group and eth-
ylidene group adjacent to the ester oxygen.32 This general
mechanism of photodegradation has been expanded by Boc-
chini et al.37 to include the formation of anhydride groups
when the radiation source used is closer to a natural outdoor
exposure. In the proposed mechanism, anhydride groups are
formed via initial tertiary radicals that react with oxygen and
extract hydrogen-forming hydroperoxide groups. The hydroper-
oxide further undergoes photolysis with the formation of the
anhydride group by b-scission.
the first route, it was found that DBU could not be success-
fully applied as a catalyst for ROP of lactide. However,
Sn(Oct)2 catalyst works well in this system, resulting in the
ONB-PDLA macroinitiator. The well-defined block copoly-
mer PDLA-ONB-PMMA was synthesized via chain extension
of ONB-PDLA by ATRP of MMA. For the second route,
although the nitrobenzene group of ONB perturbs ATRP of
MMA, the obtained ONB-PMMA macroinitiators with narrow
Ð effectively initiate ROP of lactide using Sn(Oct)2 or DBU
catalysts, resulting in the formation of PMMA-ONB-PDLA
block copolymers. The photocleavage of the copolymers
occurs under mild conditions by simple irradiation with UV
light (302 nm, relative intensity at 7.6 cm: 1500 lW/cm2)
for several hours. Moreover, it is found that PDLA
blocks also photodegraded with a corresponding reduction
of molecular weight observed by GPC and NMR which is
a further benefit for the facile removal of the sacrificial
block.
Additionally, the photocleavage was followed by 1H NMR. In
Figure 7, significant changes in the spectrum are observed
after photocleavage. The benzyl CH2 protons from the nitro-
benzyl group at d 5.5 ppm disappear and a resonance corre-
sponding to the coproduct benzaldehyde proton appears at d
10.2 ppm, indicating that the photolysis took place. At the
same time, the aromatic signals are also affected owing to the
transition from a nitrobenzene to a nitrosobenzene structure,
and thus confirming the cleavage. Furthermore, comparison of
the integral ratio of the methoxy protons in the PMMA block
and the methine proton in the PDLA block before and after
UV irradiation shows a reduction of PDLA content.
ACKNOWLEDGMENTS
Financial support from the German Science Foundation (DFG)
under Grant TH 1104/4-1 and an International Collaboration
in Chemistry award from the National Science Foundation (CHE
0924435) are gratefully acknowledged.
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